1. Field of the Invention
The present invention relates generally to composite materials. In particular, it relates to a low pressure process to prepare consolidated polyimide composite laminates.
2. Description of the Related Art
Compression molding of flat panels of fiber reinforced resin matrix composite laminates is the simplest form of a molding process which employs matched metal dies. In this process, individual prepreg plies are cut into the desired dimensions from a flat sheet of prepreg material. The number of prepreg plies used in this process is dependent on the desired final part thickness. The plies are stacked inside the cavity of the female mold and subjected to heat and compression forces to undergo consolidation. Ohta et al. ("Improved Melt Flow and Physical Properties of Mitsui Toatsu's LaRC-TPI #1500 Series Polyimide", 35th International SAMPE Symposium and Exhibition, Vol. 35, pp 1030-1044, Apr. 2-5, 1990.) reported using pressures of 2-7 MPa (284-994 p.s.i.) to form composites from LaRC-TPI.
Compression molding of a composite laminate is by no means a trivial process. Processing parameters such as heat transfer phenomena, resin flow behavior, fiber-resin interface wetting, adhesion and viscoelastic behavior, volatile escape mechanism, and bulk consolidation are just a few of the issues which need to be understood and controlled. For thermoplastic resins, resin flow behavior is one of the major issues involved in the compression molding process. The high viscosity of the resin creates problems in the wetting and penetration of reinforcing fibers, bonding of adjacent plies and removal of voids during lamination of plies, and resin flow during forming. Some forming processes are so rapid that there is insufficient time for all the entrapped air and/or volatile by-products in a prepreg stack to be expelled. Using conventional molding technology, void-free and well consolidated composite laminates are often difficult to achieve.